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//===--- RDFDeadCode.cpp --------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// RDF-based generic dead code elimination.
#include "RDFDeadCode.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/RDFGraph.h"
#include "llvm/CodeGen/RDFLiveness.h"
#include "llvm/Support/Debug.h"
#include <queue>
using namespace llvm;
using namespace rdf;
// This drastically improves execution time in "collect" over using
// SetVector as a work queue, and popping the first element from it.
template<typename T> struct DeadCodeElimination::SetQueue {
SetQueue() : Set(), Queue() {}
bool empty() const {
return Queue.empty();
}
T pop_front() {
T V = Queue.front();
Queue.pop();
Set.erase(V);
return V;
}
void push_back(T V) {
if (Set.count(V))
return;
Queue.push(V);
Set.insert(V);
}
private:
DenseSet<T> Set;
std::queue<T> Queue;
};
// Check if the given instruction has observable side-effects, i.e. if
// it should be considered "live". It is safe for this function to be
// overly conservative (i.e. return "true" for all instructions), but it
// is not safe to return "false" for an instruction that should not be
// considered removable.
bool DeadCodeElimination::isLiveInstr(NodeAddr<StmtNode *> S) const {
const MachineInstr *MI = S.Addr->getCode();
if (MI->mayStore() || MI->isBranch() || MI->isCall() || MI->isReturn())
return true;
if (MI->hasOrderedMemoryRef() || MI->hasUnmodeledSideEffects() ||
MI->isPosition())
return true;
if (MI->isPHI())
return false;
for (auto &Op : MI->operands()) {
if (Op.isReg() && MRI.isReserved(Op.getReg()))
return true;
if (Op.isRegMask()) {
const uint32_t *BM = Op.getRegMask();
for (unsigned R = 0, RN = DFG.getTRI().getNumRegs(); R != RN; ++R) {
if (BM[R/32] & (1u << (R%32)))
continue;
if (MRI.isReserved(R))
return true;
}
}
}
return false;
}
void DeadCodeElimination::scanInstr(NodeAddr<InstrNode*> IA,
SetQueue<NodeId> &WorkQ) {
if (!DFG.IsCode<NodeAttrs::Stmt>(IA))
return;
if (!isLiveInstr(IA))
return;
for (NodeAddr<RefNode*> RA : IA.Addr->members(DFG)) {
if (!LiveNodes.count(RA.Id))
WorkQ.push_back(RA.Id);
}
}
void DeadCodeElimination::processDef(NodeAddr<DefNode*> DA,
SetQueue<NodeId> &WorkQ) {
NodeAddr<InstrNode*> IA = DA.Addr->getOwner(DFG);
for (NodeAddr<UseNode*> UA : IA.Addr->members_if(DFG.IsUse, DFG)) {
if (!LiveNodes.count(UA.Id))
WorkQ.push_back(UA.Id);
}
for (NodeAddr<DefNode*> TA : DFG.getRelatedRefs(IA, DA))
LiveNodes.insert(TA.Id);
}
void DeadCodeElimination::processUse(NodeAddr<UseNode*> UA,
SetQueue<NodeId> &WorkQ) {
for (NodeAddr<DefNode*> DA : LV.getAllReachingDefs(UA)) {
if (!LiveNodes.count(DA.Id))
WorkQ.push_back(DA.Id);
}
}
// Traverse the DFG and collect the set dead RefNodes and the set of
// dead instructions. Return "true" if any of these sets is non-empty,
// "false" otherwise.
bool DeadCodeElimination::collect() {
// This function works by first finding all live nodes. The dead nodes
// are then the complement of the set of live nodes.
//
// Assume that all nodes are dead. Identify instructions which must be
// considered live, i.e. instructions with observable side-effects, such
// as calls and stores. All arguments of such instructions are considered
// live. For each live def, all operands used in the corresponding
// instruction are considered live. For each live use, all its reaching
// defs are considered live.
LiveNodes.clear();
SetQueue<NodeId> WorkQ;
for (NodeAddr<BlockNode*> BA : DFG.getFunc().Addr->members(DFG))
for (NodeAddr<InstrNode*> IA : BA.Addr->members(DFG))
scanInstr(IA, WorkQ);
while (!WorkQ.empty()) {
NodeId N = WorkQ.pop_front();
LiveNodes.insert(N);
auto RA = DFG.addr<RefNode*>(N);
if (DFG.IsDef(RA))
processDef(RA, WorkQ);
else
processUse(RA, WorkQ);
}
if (trace()) {
dbgs() << "Live nodes:\n";
for (NodeId N : LiveNodes) {
auto RA = DFG.addr<RefNode*>(N);
dbgs() << PrintNode<RefNode*>(RA, DFG) << "\n";
}
}
auto IsDead = [this] (NodeAddr<InstrNode*> IA) -> bool {
for (NodeAddr<DefNode*> DA : IA.Addr->members_if(DFG.IsDef, DFG))
if (LiveNodes.count(DA.Id))
return false;
return true;
};
for (NodeAddr<BlockNode*> BA : DFG.getFunc().Addr->members(DFG)) {
for (NodeAddr<InstrNode*> IA : BA.Addr->members(DFG)) {
for (NodeAddr<RefNode*> RA : IA.Addr->members(DFG))
if (!LiveNodes.count(RA.Id))
DeadNodes.insert(RA.Id);
if (DFG.IsCode<NodeAttrs::Stmt>(IA))
if (isLiveInstr(IA) || DFG.hasUntrackedRef(IA))
continue;
if (IsDead(IA)) {
DeadInstrs.insert(IA.Id);
if (trace())
dbgs() << "Dead instr: " << PrintNode<InstrNode*>(IA, DFG) << "\n";
}
}
}
return !DeadNodes.empty();
}
// Erase the nodes given in the Nodes set from DFG. In addition to removing
// them from the DFG, if a node corresponds to a statement, the corresponding
// machine instruction is erased from the function.
bool DeadCodeElimination::erase(const SetVector<NodeId> &Nodes) {
if (Nodes.empty())
return false;
// Prepare the actual set of ref nodes to remove: ref nodes from Nodes
// are included directly, for each InstrNode in Nodes, include the set
// of all RefNodes from it.
NodeList DRNs, DINs;
for (auto I : Nodes) {
auto BA = DFG.addr<NodeBase*>(I);
uint16_t Type = BA.Addr->getType();
if (Type == NodeAttrs::Ref) {
DRNs.push_back(DFG.addr<RefNode*>(I));
continue;
}
// If it's a code node, add all ref nodes from it.
uint16_t Kind = BA.Addr->getKind();
if (Kind == NodeAttrs::Stmt || Kind == NodeAttrs::Phi) {
append_range(DRNs, NodeAddr<CodeNode*>(BA).Addr->members(DFG));
DINs.push_back(DFG.addr<InstrNode*>(I));
} else {
llvm_unreachable("Unexpected code node");
return false;
}
}
// Sort the list so that use nodes are removed first. This makes the
// "unlink" functions a bit faster.
auto UsesFirst = [] (NodeAddr<RefNode*> A, NodeAddr<RefNode*> B) -> bool {
uint16_t KindA = A.Addr->getKind(), KindB = B.Addr->getKind();
if (KindA == NodeAttrs::Use && KindB == NodeAttrs::Def)
return true;
if (KindA == NodeAttrs::Def && KindB == NodeAttrs::Use)
return false;
return A.Id < B.Id;
};
llvm::sort(DRNs, UsesFirst);
if (trace())
dbgs() << "Removing dead ref nodes:\n";
for (NodeAddr<RefNode*> RA : DRNs) {
if (trace())
dbgs() << " " << PrintNode<RefNode*>(RA, DFG) << '\n';
if (DFG.IsUse(RA))
DFG.unlinkUse(RA, true);
else if (DFG.IsDef(RA))
DFG.unlinkDef(RA, true);
}
// Now, remove all dead instruction nodes.
for (NodeAddr<InstrNode*> IA : DINs) {
NodeAddr<BlockNode*> BA = IA.Addr->getOwner(DFG);
BA.Addr->removeMember(IA, DFG);
if (!DFG.IsCode<NodeAttrs::Stmt>(IA))
continue;
MachineInstr *MI = NodeAddr<StmtNode*>(IA).Addr->getCode();
if (trace())
dbgs() << "erasing: " << *MI;
MI->eraseFromParent();
}
return true;
}